The present invention generally relates to the acquisition of waveform data and more particularly to an apparatus, such as a time domain reflectometer, for acquiring waveform data representative of a signal from a metallic transmission cable under test.
A time domain reflectometer (TDR) launches interrogating energy pulses into a transmission medium, such as shielded and unshielded twisted pairs, coaxial cables, and the like, at a given pulse repetition rate, depending upon the designated range of the TDR. During the periods between pulses, acquisition circuitry samples the cable to acquire data representative of reflections from flaws, discontinuities, or breaks in the cable. The reflections in the cable are timed from the time of transmission of the energy pulse to determine the range from the transmitter to such flaws, discontinuities, or breaks. Reflections may represent changes in wire gauge, splices, moisture in the cable, and the like. The acquired data is processed and displayed as a waveform trace on a display device, such as a cathode-ray-tube, a liquid crystal display, or the like.
A TDR notes any changes in the characteristic impedance of the cable under test. For a telecommunications copper facility or plant, the characteristic impedance is typically between 100 and 125 ohms. Most unshielded cables fall between 100 and 105 ohms. Shielded cable like T1 is typically about 125 ohms. Any change in the cable's impedance is displayed on the TDR display device as an upward bump, downward dip, or some combination of both deviating from a horizontal trace.
The TS100 and the TV110 Time Domain Reflectometers, manufactured and sold by Tektronix, Inc., Wilsonville, Oreg. and assignee of the instant invention, are examples of TDRs for respectively characterizing telephone twisted pair cables and coaxial cable, such as in cable television systems and the like. The TV110 automatically selects the pulsewidth of the interrogating pulse and the gain of the receiver based on the initial span of the instrument. The acquisition hardware in the TV110 acquires a single data point for each launched interrogating pulse. Timing circuitry in the acquisition hardware varies the sample strobe timing in relation to the launched interrogating pulse to acquire data points at different time intervals to build a waveform data set over the selected transmission cable span. Increasing the span causes the instrument to select a new pulsewidth and gain and to reacquire new data samples over the new range. The acquired waveform data set is displayed on a liquid crystal display device and movable cursors are provided for locating the position of displayed faults.
The TV110 acquires the waveform data at a single receiver gain setting. There is the possibility that the preprogrammed gain settings would not be large enough to detect small amplitude return events in the transmission cable. In addition, each change in span setting requires the re-acquisition of the complete waveform record over the new span. Further, an operator is required to view the displayed waveform and, based on experience, move a cursor to the location of a displayed event and place the cursor at what the user believes to be the start of the event to determine the location of the event.
U.S. Pat. No. 5,461,318 describes a time domain reflectometer having a pulse generator that generates variable amplitude interrogating pulses based on cable length. The amplitude of the pulse is dependent upon the distance to the horizontal sample being taken. If the distance is less than 1,000 feet, the transmitted pulse is 5 volts. If the distance is 1,000-10,000 feet the pulse amplitude is 7.5 volts and for a distance greater than 10,000 feet the pulse amplitude is 10 volts. The pulse height is made changeable "on the fly" such that as the distance becomes progressively longer, the transmitted pulse becomes progressively higher in amplitude, yet is limited in amplitude so as not to overdrive the TDR sampler and A/D converter. The purpose of this technique is the same as increasing the pulse width of the interrogating pulse with distance, mainly to increase the injected energy into the cable with distance. However, there is no assurance that increasing the injected energy into the cable with distance will increase the likelihood that small events will be detected in the cable.
In the field of optical time domain reflectometry (OTDR), automated waveform acquisition and event detection and characterization has progressed farther than with metallic time domain reflectometers. U.S. Pat. No. 5,155,439, assigned to the assignees of the present invention describes an optical fault locator that launches optical pulses into a test fiber at a first pulse width. The return reflected optical signal is converted to an electrical signal, digitized, stored and processed to locate anomalous events in the fiber. Any region of the test fiber having an anomalous event is reexamined using optical pulses having a pulse width optimized for the that region of the fiber. A symbolic display is used to indicate the location and type of event located in the fiber instead of the traditional waveform trace.
U.S. Pat. No. 5,528,356 describes an OTDR that acquires and stores waveform data points having multiple waveform segments with each waveform segment having data points acquired using different pulsewidths, sample spacing and starting distance. The waveform segment is defined in terms of the noise floor. The gain of the OTDR receiver amplifier may be increased for the various waveform segment acquisitions in conjunction with other parameters, such as the pulsewidth, averaging and the like, to increase the signal to noise ratio within the segment.
There are significant differences between optical time domain reflectometry and metallic time domain reflectometry. In optical time domain reflectometry, the return signal from the fiber under test is generated by Rayleigh backscattering. This means that there is a continuous return signal from the fiber even with a clean fiber having no events. Discontinuities from such things as optical connectors in the fiber appear as high amplitude positive reflective events. Reflectionless losses in the fiber appear as changes in the backscatter level. Because the return reflections and the reflectionless losses are always positive, the operating point of the AID converter is set toward the lower limit of the device. In metallic time domain reflectometry, there is no corresponding Rayleigh backscattering as in optical time domain reflectometry. This means that a clean cable having no events would not generate a return signal. In addition, return reflections from a metallic cable may be either positive or negative based on the type of event. This requires the operating point of the A/D converter to be set at the midpoint of the AID dynamic range. This requirement essentially decreases the dynamic range of the A/D converter in a metallic TDR to one-half of that of an OTDR. Because of the limited dynamic range of the A/D converter in a metallic TDR, it has proved difficult to detect and characterize all the events in the acquired waveform data from the cable under test return signal.
What is needed is a time domain reflectometer that overcomes the A/D converter dynamic range limitations of current time domain reflectometers for acquiring waveform data representative of the return signal from a cable under test. The time domain reflectometer should acquire the waveform data in such a manner as to allow the detection and characterizations of events with low return signals. The time domain reflectometer should also acquire the waveform data over a segment length or multiple segment lengths of the cable using optimized acquisition parameters for each cable segment length. The time domain reflectometer should further display a representation of the cable under test showing the where the event occur in the cable and characterizing data on the events.